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408                              CHEMICAL ENGINEERING
solid form. Perhaps the most familiar example is the "salting out" of soap, where common salt is added to thick soap solution in the boiling kettle, causing the soap to separate from the water and lye and to collect in dense masses which can be worked up as desired. An even better example is the case of some of the nitro compounds which are soluble in the strong acids in which the nitration takes place but upon drowning the acid in water appear as small but well denned crystals. A very similar case is the precipitation of organic compounds from solution in fine crystalline form by addition of solvent which dissolves in all proportions in the solvent holding the desired material but in which the material itself is not soluble, i.e., the precipitation of pure sugars from water solution by strong alcohol.
Another case is the precipitation of silver nitrate from strong nitric-acid solutions, and of barium chloride from strong hydrochloric-acid solutions.
Notwithstanding the vast production of crystalline products and the great amount of work which has been done on crystallization, our knowledge is still to a large degree empirical. If the laws of physical chemistry are applied to crystalline chemical products it is possible to get better and more uniform results than are now generally obtained, especially in crystallization from solutions of several components. The practical application of these laws will of course require close control and more expensive equipment and means for maintaining temperatures, but the results will be more than commensurate. An excellent example of what is possible in this direction has been the development of the Freeth or Brunner-Mond process of the manufacture of ammonium nitrate from ammonium sulphate and sodium nitrate. In this process the four-component system (NHJaSCU  NaNO3  NH4N03  Na2S04 was studied and worked out in the laboratory by the phase rule and the exact concentrations and temperatures necessary to bring about the transposal were determined and then put in practice. The plant erected in this country for supplying the need of ammo-niumnitrate during the war had a capacity of 600,000 Ib. per day-of ammonium nitrate of 96 to 97 per cent purity. A general outline of the process is as follows: A hot solution of ammonium sulphate of exactly the predetermined density was made and to it was added a calculated quantity of sodium nitrate. With thorough mixing and cooling to the exact extent necessary the bulk of the sodium sulphate formed and separated out and was removed from the solution by continuous niters. The nitrate was diluted to the density for the proper crystallization of the ammonium nitrate and run into long shallow crystallizing pans arranged in tiers in an immense room the air of which was constantly circulated and kept at exact temperature and moisture content which had been determined as most favorable for ammonium-nitrate crystallization. When the proper time for crystallization had elapsed, the mother liquor was drawn from the pans and the crystals shovelled out, dried, and shipped.
The sodium sulphate crystals were washed with a little water and again filtered, the crystals being dumped out and the wash going to join the mother liquor. This mixed liquor was concentrated to the proper point to receive more ammonium sulphate and another charge of sodium nitrate, and the operation continued. The crystallizing rooms contained some miles of pans and had the largest air-conditioning plant ever erected. It has been stated that by the addition of a few units in one or two steps of the process the capacity could readily have been increased to 800,000 Ib. per day.
A proper application of phase rule control would greatly simplify the problem of handling impure solutions such as described under ferrocyanides, raw brines etc., arid doubtless many others which it has not been attempted to discuss. It seems to be -a method of wide potential application but necessitates considerable laboratory work.1
lln connection with this chapter one should also study Sec. IX on leaching and dissolving; Sec. X on evaporation; Sec. XXVI, the portion covering the preparation by crystallization of radium compounds; the tables of solubilities in the appendix and the BaumeVBrix-Twaddell-specific gravity conversion tables on pp. 397 and 398.